Low noise electric switch



May 24, 1960 I w.- R. NAss v 2,938,094

' Low NOISE ELECTRIC SWITCH Filed Dec. 24. 1956 '2 Sheets-Sheet 1 c/CL Es /HUY/vD/QEDS) Flu BYMTW YY'OPNEK May 24,- 1960 W. R. NASS Filed Dec. 24, 1956 LOW NOISE ELECTRIC SWITCH 2 Sheets-Sheet 2 United States Patent O LOW NOISE ELECTRIC SWITCH Walter R. Nass, Escondido, Calif., assignor to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware Filed Dec. 24, 1956, Ser. No. 630,245 2 Claims. (Cl. 200-164) This invention relates to low noise switches and more particularly to a switch which introduces a minimum of noise into the circuit when subjected to severe shock and vibration.

Most electric switches have two contacts, one movable by spring means and the other stationary. An arm may be moved to engage a contact and restrained by spring means or a commutator may be rotated to engage a spring urged brush. The mass of the arm or brush and 'the characteristics of the spring provide a natural resonant frequency, usually within the 50-150 cycles per second region. When these switches are subjected to vibration testing and the frequency of the switch vibration approaches the resonant frequency of the switch elements, a momentary opening of the switch contacts will occur. YThis will cause an open circuit if the opening is large enough and the current low enough that arcing does not occur. If the spacing between the contacts is small enough it will not cause an open circuit but will cause a variation in contact resistance. This results in noise in the circuit signal. In precision circuitry such as in rocket and missile design either the open circuit or change in contact resistance will cause noise of greater than 1.5 millivolts, which is above the tolerance permitted in the circuit design. The use of heavier and stronger springs will not solve the problem. This merely changes the resonance of the contact to a higher frequency and when `vibration of the switch reaches the higher frequency the same diculty is again experienced.

The low noise switch comprising the present invention includes a contact of heterogeneous material performing the function of both spring and brush. It has a low mass and its resonant frequency is quite high. This material consists of a mat of randomly oriented highly conductive resilient wires having a high hysteresis characteristic. Because of friction of the wires passing over each other in vibrational movement, the material has a very low Q. The wires are initially compressed in installation to provide a large contact surface and a desired low switch resistance. The switch is balanced so that the friction of the wires against the commutator prevent accidental switch operation due to vibration.

It is therefore an object of this invention to provide for an improved low noise switch.

Another object is the provision of a switch contact of W mass and high resonant frequency.

Another object is the provision of a switch contact of high hysteresis and low Q characteristics due to its internal friction.

Another object is the provision of a switch which, when subjected to 16 Gs from 0 to 2,000 cycles, has a Variation of less than 1.5 millivolts in a 20 ampere current circuit.

Another object is the provision of a switch wherein the mechanical resonance characteristics may be conveniently adjustable.

Still another object is the provision of a switch contact of heterogeneous material having resiliency and high conductivity characteristics.

ICC

Other objects and features of the present invention will be readily apparent to those skilled in the art from the following specification and appended drawings wherein is illustrated a preferred form of the invention, and in which:

Figure 1 is a perspective of the low noise switch, which also uses conventional brushes for part of the contacts; Figure -2 is a cross-sectional view of the switch, having conventional brushes as well as low noise brushes;

Figures 3 shows noise curves of the invention switch and a conventional switch; and

Figure 4 is a schematic view of one application of the switch.

Referring now to Figure l, a'plurality of switch contacts 11 and 12 are spaced axially and radially about a commutator having electrical paths thereon. As the commutator is rotated these paths simultaneously connect various of the contacts and completing circuitry connected thereto.

Since the external circuitry is not considered as part of this invention except in connection therewith, circuitry detail is not shown. Contacts 11 are of conventional design as will be seen with reference to Figure 2 and contacts 12 are those original with the present inventor. The conventional contacts 11 were used for test comparison purposes and later replaced with contacts 12. Motors 13 and 14 are connected to the commutator Yfor driving it in either direction in an amount and speed determined by external circuitry connected thereto. The switch and motors are mounted on Ybrackets 16 and`17 which in turn are fastened to mounting plate 18.

Figure 2 is a cross-sectional view of the switch taken along the commutator axis. The commutator 19 consists of a rotor 21 mounted for rotation in suitable bearings 22 and 23 on brackets 16 and 17. Couplings 24 and 26 connect the rotor 21 to motors 13 and 14 for actuation in either direction depending upon which motor is energized. Mounted on rotor 21 and adapted to rotate therewith is a sleeve 27 of nonconducting material. On the surface of the sleeve are recesses filled with conducting material 28. Each of these recesses lled with material is adapted to be contacted by suitable brush contacts during a part of the commutator rotation. Also certain recesses may be electrically connected to others in a prearranged manner to connect certain groups of external circuits as desired. The electrical paths thus formed on the surface of sleeve 27 may also be made by printing or painting with electrically conducting paint or ink in the conventional manner. The outer surface of sleeve 27, however, must'be smooth and cylindrical in shape and of uniform radius from its axis of rotation to present a constant surface to the contacting brush.

Within housing 29, within which the commutator rotates, is mounted a plurality of radial rows of contacts 11 and 12. Contacts 12 are of conventional design and consist of silver graphite brushes 31 with 1A diameter springs 32 applying one pound pressure against the brushes. Pins 33 are electrically connected to the brushes 12 for providing terminal connections therefor. Contact 11 consists of a plug 34 threadedly engageable with housing. This plug is electrically conducting and has a terminal connection 36 attached thereto. This plug is shorter than the thickness of the housing and between the inner surface of the plug and the inner surface of the housing is sandwiched a contact of heterogeneous material 37 which performs the function of both brush 31 and supporting spring 32. In the preferred embodiment this material consists of silver plated copper wool threads randomly oriented. However other materials such as beryllium copper, silver plated steel, molybdenum and materials of similar characteristics may be used if desired. Because of friction of the threads passing over each other during vibrational movement, the material has a very low Q and a high hysteresis characteristic. These wires are initially compressed by plug 34 upon installation to provide a good surface contact with the electrically conducting portion 28 of commutator 19. In the preferred embodiment the material 37 was tightened by plug 34 until there was a static brush drop of 25 millivolts when energized with a 20 ampere current. The rotor was turned several times to wear in the contact and to provide a burnished contact surface for maximum conductivity. y

Figure 3 is a chart comparing the performance of contact 11 with the conventional contact 12 when vibrated in a lateral direction from to 2,000 cycles per second. The solid line 38 represents the noise level of `the conventional contact l2 and the dotted line 39 represents the noise level of contact 11. As shown by the chart the silver graphite brush has noise spikes of 100, Y150, 20D-300, 600 and 100 millivolts at 50, 123, 210-250, 350 and 400 cycles respectively and between 500'and 2,000 cycles the noise level is within the l0 millivolt maximum desired for the circuit with `which' this switch is intended. The silver coated copper Wool, when subjected to the same conditions, had noise spikes of l0, 60, 20, 20 and 0-l0 millivolts at 320, 700, 940,*1060 and 1100-2000 cycles respectively. At other frequencies the noise level remained at 0.

Figure 4 is a schematic of one application of the switch. Here the conimutator i9 has a conducting s1 vface 41 extending around its circumference so that all three contacts 42, 43, 44 are electrically connected thereby. Connected to contact 42 is a power supply of a certain type, shown as battery 46, and connected to contact 44 is a diiferent power supply, shown as generator 47. Center contact 43 is the switch output and may be connected to suitable external circuitry. When the commutator 19 is rotated clockwise the external circuit is energized by power supply 47 and when rotated counterclockwise, the circuit is energized from power supply 46. Here the switch is make-before-break so that the external circuit is always energized by at least one of the power sources. The commutator is so balanced that during vibration the friction of the material 37 is sulicient to prevent accidental rotation.

While certain preferred embodiments of the invention have been specically disclosed, it is understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

What I claim is: Y

l. A rotatable switch comprising a housing, a commutator rotatably mounted in said housing, means for rotating said commutator in Aeither direction, said commutator having electrically conducting portions thereon, openings in said housing adjacent said commutator and in alignment with the path of rotation of said conducting portions, a quantity of randomly oriented electrically conductive threads in said openings and contacting said commutator, an electrically conductive plug inserted in each of said openings for connecting lead lines thereto, said plug being adapted to adjustably compress said quantity of threads against said commutator to prevent electrical noise during vibration and to remove its resonant frequencies beyond the range of expected environmental frequencies, said threads frictionally engaging each other to frictionally resist resonant frequency disturbance, said threads frictionally engaging said commutator while under compression to prevent accidental movement of said comm-utator during vibration.

2. A power transfer switch for selectively energizing an external circuit from either of two power sources comprising a housing, a commutator rotatably mounted within said housing, a plurality of contact means on said housing contacting said commutator, said pluralityr'of contact means being in axial alignment with and radially spaced about said commutator, the outermost contact means each being connected to a separate power source, the central contact means being connected to an external circuit, said commutator having an electrical path thereon in alignment With said plurality of contact means and extending around said commutator a distance at least equal to the circumferential spacing of said outermost contact means, said contact means comprising a Vquantity of randomly oriented electrically conductive threads frictionally engaging said commutator, means for adjustably compressing said threads against said commutator to prevent electrical noise during vibration to remove its resonant frequencies beyond the range of expected environmental frequencies, and means for turning said commutator a predetermined amount in either direction whereby at least one of said power sources is always in electrical contact with said central contact means.

References Cited in the tile of this patent UNITED STATES PATENTS 1,999,981 Ruger Apr. 30, 1935 2,487,187 Seifried et al. Nov. 8, i949 2,520,709 Bitler Aug. 29, 1950 2,866,047 Stuellein et al. Dec. 23, 1958 

